The present technique relates generally to the field of electrical distribution. Specifically, the invention relates to techniques for determining the impedance parameters of electrical power, for determining incident energy, for determining a flash protection boundary, and for determining a level of personal protective equipment (“PPE”) that may be required or advisable based upon the available energy and similar considerations.
Systems that distribute electrical power for residential, commercial, and industrial uses can be complex and widely divergent in design and operation. Electrical power generated at a power plant may be processed and distributed via substations, transformers, power lines, and so forth, prior to receipt by the end user. The end user may receive the power over a wide range of voltages, depending on availability, intended use, and other factors. In large commercial and industrial operations, the power may be supplied as three phase ac power (e.g., 208 to 690 volt ac, and higher). Power distribution and control equipment then conditions the power and applies it to loads, such as electric motors and other equipment. In one exemplary approach, collective assemblies of protective devices, control devices, switchgear, controllers, and so forth are located in enclosures, sometimes referred to as “motor control centers” or “MCCs”. Though the present techniques are discussed in the context of MCCs, the techniques may apply to power management systems in general, such as switchboards, switchgear, panelboards, pull boxes, junction boxes, cabinets, other electrical enclosures, and distribution components.
A typical MCC may manage both application of electrical power, as well as data communication, to the loads, such loads typically including various machines or motors. A variety of components or devices used in the operation and control of the loads may be disposed within the MCC. Exemplary devices contained within the MCC are motor starters, overload relays, circuit breakers, and solid-state motor control devices, such as variable frequency drives, programmable logic controllers, and so forth. The MCC may also include relay panels, panel boards, feeder-tap elements, and the like. Some or all of the devices may be disposed within units sometimes referred to as “buckets” that are mounted within the MCC. The MCC itself typically includes a steel enclosure built as a floor mounted assembly of one or more vertical sections containing the buckets.
The MCC normally contains power buses and wiring that supply power to the buckets and other components. For example, the MCC may house a horizontal common power bus that branches to vertical power buses within the MCC. The vertical power buses, known as bus bars, then extend the common power supply to the individual buckets. Other large power distribution equipment and enclosures typically follow a somewhat similar construction, with bus bars routing power to locations of equipment within the enclosures.
To electrically couple the buckets to the vertical bus, and to simplify installation and removal, the buckets may comprise electrical connectors or clips, known as stabs. To make the power connection, the stabs engage (i.e., clamp onto) the bus bars. For three phase power, there may be at least three stabs per bucket to accommodate the three bus bars for the incoming power. It should be noted that though three phase ac power is primarily discussed herein, the MCCs may also manage single phase or dual phase ac power, as well as dc power (e.g., 24 volt dc power for sensors, actuators, and data communication). Moreover, in alternate embodiments, the individual buckets may connect directly to the horizontal common bus by suitable wiring and connections. Similarly, in contexts other than MCCs, the structures described herein will, of course, be adapted to the system, its components, and any enclosures that house them.
A problem in the operation of MCCs and other power management systems, such as switchboards and panelboards, is the occurrence of arcing (also called an arc, arc fault, arcing fault, arc flash, arcing flash, etc.) which may be thought of as an electrical conduction or short circuit across the air between two conductors. Initiation of an arc fault may be caused by a loose connection, build-up of foreign matter such as dust or dirt, insulation failure, or a short-circuit between the two conductors (e.g., a foreign object establishing an unwanted connection between phases or from a phase to ground) which causes the arc. Once initiated, arcing faults often typically proceed in a substantially continuous manner until the power behind the arc fault is turned off. However, arcing faults can also comprise intermittent failures between phases or phase-to-ground. In either case, the result is an intense thermal event (e.g., temperatures up to 35,000° F.) causing melting or vaporization of conductors, insulation, and neighboring materials.
The energy released during an arcing fault is known as incident energy. Incident energy is measured in energy per unit area, typically Joules per square centimeter (J/cm2). Arcing faults can cause damage to equipment and facilities and drive up costs due to lost production. More importantly, the intense heat generated by arcing faults has led to the establishment of standards for personal protective equipment (“PPE”) worn by service personnel when servicing electrical equipment.
There are five levels of PPE numbered from 0 to 4. Whereas, level 0 PPE comprises merely a long sleeved shirt, long pants, and eye protection, level 4 PPE comprises a shirt, pants, a flame retardant overshirt and overpants, a flash suit, a hard hat, eye protection, flash suit hood, hearing protection, leather gloves, and leather work shoes. PPE levels 1–3 comprise increasing amounts of protective clothing and equipment in increasing greater amounts between levels 0 and 4. As such, the higher the PPE level, the more protective clothing or equipment a person will put on (referred to as “donning”) or take off (known as “doffing”) before servicing the equipment. Accordingly, the time to donn and doff the protective equipment increases as the PPE level increases. For example whereas it may take less than a minute to donn or doff level 1 PPE, it may take 20 minutes or more to donn or doff level 4 PPE. These donning and doffing times can directly affect productivity. As such, it is advantageous to accurately determine the potential incident energy of a potential arc flash so that the appropriate level of PPE.
Many other uses and applications exist for information relating to incident energy, and other power line electrical parameters. These include, but are not limited to, the sizing and design of filters, the commissioning and design of motor drives and other equipment, the monitoring of power lines and components for degradation and failure, and so forth.
Conventional methods for determining power line parameters and PPE levels rely on approximation techniques or require complex, extensive modeling of electrical equipment. There is a need in the art for improved techniques for determining the incident energy. There is a particular need for a technique that would permit the accurate determination and communication of incident energy, the determination of flash protection boundaries, and the determination and communication of PPE levels that correspond to a particular incident energy.